In the said injection mold based on the conventional technology, as shown in FIG. 5, a core 22 is movably provided inside a rigid cavity 21 between a closing position and an opening position of the mold, synthetic resin is filled via a gate 24 of a cavity 21 in a gap 23 between the cavity 21 and the core 22, the synthetic resin is kept and cooled in a pressurized state, then the core 22 moves to the mold opening position, and the molded product 25 is taken out from the core 22.
In the injection mold as described above, when a molded product with a large flow length in the mold opening direction, namely with a large flow ratio of a flow length/thickness is molded, increasing thickness when filling is effective to make it easier to fill synthetic resin in the gap 23. The method of utilizing a mold opening to achieve the objective is well known, but in this method, there is the problem that, assuming the mold opening rate DY as shown in FIG. 5, thickness of a bottom surface of a molded product 25 increases by DY, but thickness of a side surface only by DY sin .theta., namely that, assuming .theta. of 5 .degree. and DY of 1 mm, thickness of a bottom surface is 1 mm, but that of a side face is 0.09 mm, which is too thin to achieve the thickness effect.